1. Field of the Invention
The present invention relates to an evaporated fuel treatment device, and more particularly to an evaporated fuel treatment device for treating evaporated fuel generated in a fuel tank without emitting it to the atmosphere.
2. Background Art
A conventional evaporated fuel treatment device disclosed, for instance, by JP-A No. 2001-342914 is equipped with a canister that communicates with a fuel tank. This device is also equipped with a purge path for introducing an intake negative pressure into the canister as well as a bypass path that is positioned between the fuel tank and canister for introducing a negative pressure into the fuel tank. The bypass path is provided with a bypass control valve, which controls the continuity of the bypass path.
If an open failure occurs in the bypass control valve of the above conventional device, the continuity between the canister and the fuel tank cannot be cut so that normal operations cannot be assured. Therefore, the above conventional device has a function for detecting an open failure in the bypass control valve by a method described below.
More specifically, when the above conventional device needs to detect an open failure in the bypass control valve, it first issues a valve close instruction to the bypass control valve while introducing an intake negative pressure into the canister. Next, the conventional device monitors a canister internal pressure and tank internal pressure to check whether the tank internal pressure significantly follows a change in the canister internal pressure.
If the bypass control valve is properly closed, the bypass control valve shuts off the intake negative pressure introduced into the canister. In this instance, therefore, the tank internal pressure does not follow the canister internal pressure. If, on the other hand, the bypass control valve is open in spite of the issued valve close instruction, the intake negative pressure introduced into the canister is also introduced into the fuel tank. As a result, the tank internal pressure significantly follows a change in the canister internal pressure.
Therefore, if the tank internal pressure does not significantly follow a change in the canister internal pressure, the above conventional device concludes that the bypass control valve is normal. If, on the other hand, the tank internal pressure significantly follows a change in the canister internal pressure, the above conventional device concludes that an open failure exists in the bypass control valve. As described above, the foregoing conventional device is capable of judging in accordance with the changes in the canister internal pressure and tank internal pressure whether an open failure exists in the bypass control valve.
In the above conventional device, however, the tank internal pressure varies not only with the introduction of intake negative pressure but also with fuel consumption and evaporated fuel generation. To accurately judge whether the tank internal pressure adequately follows a change the canister internal pressure, it is necessary to remove the influence of fuel consumption and evaporated fuel generation. In reality, therefore, it is necessary to exercise complicated control so as to yield an accurate diagnostic check result concerning an open failure in the bypass control valve by a method employed by the above conventional device.